Best Generator for a Furnace: What Size Runs Your Heat in an Outage

A typical gas furnace blower draws about 600 W running and about 1,800 W for the moment it starts. Our generator sizer adds a 1.2 margin to running watts and 1.1 to starting watts, so the furnace alone needs about 720 W running and about 1,980 W starting. Any generator rated 2,000 W starting or more covers it.

Add the essentials and the numbers stay small. Furnace, refrigerator (150 W), chest freezer (100 W) and four LED lamps total 886 W running, or about 1,060 W with margin. The peak comes when the blower starts with everything else already on: 886 W plus the blower's 1,200 W jump is 2,086 W, about 2,300 W with margin. A 2,500 W inverter clears it on gasoline with little to spare; a 5,000 W unit gives you room for a microwave or a space heater in one room.

Check your own furnace. The blower's amps are on the furnace nameplate; multiply by 120 for watts. Newer variable-speed (ECM) blowers often draw a few hundred watts on low, so they use less fuel than the 600 W figure here.

A furnace control board is a small computer that runs the igniter, reads the flame sensor and times the blower. Open-frame generators without an inverter can produce power with more voltage swing and harmonic distortion, which some boards and ECM blower motors do not tolerate. An inverter generator produces clean, steady power similar to the grid.

If the furnace starts, lights for a few seconds and then shuts off on generator power, the common cause is the generator's neutral. Many inverter generators have a floating neutral, and some furnace flame sensors need a bonded neutral to read the flame. A neutral-ground bonding plug in a spare outlet fixes this on a cord and switch hookup. Do not use one when the generator feeds a panel through an interlock or transfer switch that already bonds neutral to ground.

Most furnaces are hardwired to a dedicated circuit, so there is no plug to move. A furnace transfer switch mounts beside the furnace and swaps it between house power and an inlet or cord from the generator. It is the cheapest safe way to run only the furnace, and an electrician can fit it in under an hour.

If you also want the well pump, lights and other circuits, a panel interlock or a transfer switch at the main panel feeds them all. That route needs a generator with a 240V outlet; see our interlock and transfer switch guide. Never backfeed a dryer outlet or plug a generator into a wall socket. It can electrocute line workers and burn out the furnace.

Run the generator outside, at least 20 feet from doors and windows, with the exhaust pointing away from the house. Put a battery CO alarm near the furnace and in sleeping areas.

On a well, the furnace is usually not the hard load; the 240V well pump is. A 1/2 HP pump needs about 3,000 W to start and a 1 HP pump about 6,000 W, and neither runs on a 120V-only generator. For both, buy a generator with a 240V outlet and at least 8,000 W starting for a 1/2 HP pump, or 9,500 W for a 1 HP pump, and connect it through an interlock or transfer switch.

A power station runs a furnace indoors with no fuel, noise or CO. The math: a 600 W blower that runs half the time averages about 300 W. A 2,000 Wh power station delivers about 85 percent of its rating, roughly 1,740 Wh, so it runs the furnace for about 6 hours. Add the fridge and that drops to about 5 hours.

That covers a short outage or a cold night, not a three-day ice storm, unless you add battery packs or solar. The EcoFlow DELTA 2 Max expands to 6,144 Wh, about 17 hours of furnace runtime. The power station needs at least 1,800 W of surge for the blower start; both picks here clear it.